In an orthogonal frequency division multiplexing access (OFDMA) communication system, with regard to the downlink direction, a base station uses an entire available frequency band, and allocates sub-channels to a plurality of mobile stations, whereby signals are transmitted.
On the other hand, with regard to the uplink direction, mobile stations utilize sub-channels allocated to the respective mobile stations by a base station, and specified modulation methods and encoding ratios, whereby signals are transmitted.
By concentrating transmission power on the allocated sub-channel band alone, the mobile station is able to enhance a power spectral density, allowing its communication area to be expanded. However, if the number of sub-channels is made smaller and the sub-channel band is made narrower, a user communication speed declines.
Conventionally, there is a method of allocating sub-channels (bandwidths) based on communication requests associated with mobile stations (for example, Patent Document 1) However, in the conventional technology described in Patent Document 1, the system determines a bandwidth to be used by a mobile station such that the remaining available bandwidth is optimized. In other words, the object is to save the bandwidth as much as possible for other mobile stations to use. Accordingly, with the conventional technology, it has been impossible to effectively utilize available resources, and therefore to achieve improvement in communication speed and reduction in power consumption for the mobile station in communication.
Patent Document 1: JP 2003-513588 A
Patent Document 2: JP 2003-18117 A
Patent Document 3: JP 2004-214746 A
In such a communication system as high speed downlink packet access (HSDPA), there is adopted an adaptive modulation method in which a modulation method is changed in accordance with the wireless environment. However, with such an adaptive modulation method, the frequency bandwidth is fixed, and hence consideration is not given to the influence from the change of the frequency bandwidth, which causes increase or decrease in transmission power spectral density.
Even with the OFDMA communication system, if the number of sub-channels is made smaller, and the number of sub-channels is assigned in a fixed manner, it is possible to expand the communication area. However, in such a case, even when the wireless environment is excellent because of a base station located nearby, only the communication speed is improved owing to the change of the modulation method.
If, in addition to the modulation method, the number of sub-channels is changed dynamically, the expansion of the communication area and the improvement in communication speed can be expected. However, appropriate allocation needs to be performed with regard to the modulation method and the number of sub-channels, and hence a new allocation method is demanded.
For example, if the allocation of sub-channels is not appropriate, with a more than necessary number of sub-channels are allocated with respect to the modulation method and encoding ratio specified for the mobile station, the power spectral density transmitted from the mobile station declines, and it becomes impossible to satisfy desired communication quality with respect to the modulation method and encoding ratio specified by the base station. Conversely, if the number of allocated sub-channels is insufficient, it is impossible to realize a maximum user communication speed that is originally expected to be achieved.